1. Field of the Invention
The present invention generally relates to a balanced filter and a duplexer, and more particularly, to a balanced filter and a duplexer that include a balun.
2. Description of the Related Art
In recent years, there are cases where signals on reception sides are differential signals, so as to restrain common mode noise in high-frequency circuits. With this trend, some high-frequency devices used for reception circuits are of a differential type (a balanced type). More specifically, balanced low-noise amplifiers (LNA) and balanced mixers are used.
FIG. 1 is a block diagram of a reception circuit in the vicinity of the antenna of a portable telephone device equipped with a balanced mixer 128. A signal that is received through a common terminal (antenna terminal) Ant is amplified by a LNA 122. An unbalanced reception filter 124 eliminates unnecessary signal components. The mixer 128 converts the signal into a low-frequency signal. Since the balanced mixer 128 has a balanced input, a balun 126 for unbalanced-to-balanced conversion is connected between the unbalanced reception filter 124 and the balanced mixer 128.
FIG. 2 is a block diagram of a reception circuit in the vicinity of the antenna of a portable telephone device equipped with a balanced LNA 136 and a balanced mixer 138. A signal that is received through a common terminal (antenna terminal) Ant passes through an unbalanced antenna duplexer 132, and reaches the reception circuit. After amplified by the balanced LNA 136, the signal is converted into a low-frequency signal by the balanced mixer 138. Since the LNA 136 has a balanced input, a balun 134 for unbalanced-to-balanced conversion is connected between the unbalanced antenna duplexer 132 and the LNA 136.
A ladder filter shown in FIG. 3 is often used as the unbalanced reception filter 124 or the unbalanced antenna duplexer 132. The ladder filter is designed to have series-arm resonators S1 through S4 connected in series and parallel-arm resonators P1 and P2 connected in parallel between an input terminal In and an output terminal Out. The resonators may be surface acoustic wave resonators, boundary acoustic wave resonators, or piezoelectric thin-film resonators, for example.
FIG. 4A is a plan view of a surface acoustic wave resonator. FIG. 4B is a cross-sectional view of the surface acoustic wave resonator, taken along the line A-A of FIG. 4A. As shown in FIG. 4A, a comb-like electrode (IDT: interdigital transducer) is formed on a piezoelectric substrate 50, and reflectors R0 are formed on both sides of the IDT. An input terminal In and an output terminal Out are connected to the IDT. As shown in FIG. 4B, electrodes 52 that form the IDT and the reflectors R0 are formed with a metal film on the piezoelectric electrode 50. In the surface acoustic wave resonator, an acoustic wave excited by the IDT becomes a standing wave on the surface of the piezoelectric substrate 50, and resonates with the frequency that is determined by the pitch of the electrodes 52 of the IDT and the propagation speed of the surface acoustic wave. With this arrangement, the surface acoustic wave resonator functions as a resonator.
FIG. 5A is a plan view of a boundary acoustic wave resonator. FIG. 5B is a cross-sectional view of the boundary acoustic wave resonator, taken along the line A-A of FIG. 5A. Unlike the structure shown in FIGS. 4A and 4B, the boundary acoustic wave resonator has a dielectric film 54 and a dielectric film 56 formed on the IDT and the reflectors R0. The other aspects of this structure are the same as those of the above described surface acoustic wave resonator. In the boundary acoustic wave resonator, an acoustic wave excited by the IDT becomes a standing wave in the dielectric film 54 that is the interface layer between the piezoelectric substrate 50 and the dielectric film 56. The standing wave then resonates with the frequency that is determined by the pitch of the electrodes 52 of the IDT and the propagation speed of the boundary acoustic wave.
FIG. 6A is a plan view of a piezoelectric thin-film resonator. FIG. 6B is a cross-sectional view of the piezoelectric thin-film resonator, taken along the line A-A of FIG. 6A. A lower electrode 62 and an upper electrode 66 are formed on an insulating substrate 60, so as to interpose a piezoelectric film 64. A void 68 or a multilayer reflection film or the like exists below the lower electrode 62. When a high-frequency signal is applied between the lower electrode 62 and the upper electrode 66, a standing wave of thickness longitudinal vibration is generated in the piezoelectric film 64, and resonates with the frequency that is determined by the film thickness of the piezoelectric film 64 and the propagation speed of the thickness longitudinal vibration.
FIG. 7 shows an example of a double-mode surface acoustic wave filter that is used in an unbalanced filter or an unbalanced duplexer. In the double-mode filter, an input IDT and output IDTs are formed on the piezoelectric substrate 50, and reflectors R0 are arranged outside the IDTs. In FIG. 7, two output IDTs (Out IDT) are provided between the two reflectors R0, and an input IDT (In IDT) is provided between the two output IDTs (Out IDT). The outputs of the two output IDTs (Out IDT) are connected to an output terminal Out. The input IDT is connected to an input terminal In. In a double-mode boundary acoustic wave filter, the dielectric films 54 and 56 shown in FIG. 5B are formed on the IDTs and the reflectors R0 shown in FIG. 7.
An unbalanced antenna duplexer is formed with a transmission filter 20, a reception filter 10, and a matching circuit 30. FIG. 8 illustrates the structure of an unbalanced duplexer that includes ladder filters serving as the transmission filter 20 and the reception filter 10. The transmission filter 20 is connected between a common terminal Ant and a transmission terminal Tx. The transmission filter 20 includes series-arm resonators S21 through S24 and parallel-arm resonators P21 and P22. The reception filter 10 is connected between the common terminal Ant and a reception terminal Rx. The reception filter 10 includes series-arm resonators S11 through S14 and parallel-arm resonators P11 through P13. The matching circuit 30 is provided between the common terminal Ant and the reception filter 10.
A device that can be used as the matching circuit 30 of the duplexer shown in FIG. 8 is now described. FIG. 9A shows a strip line/microstrip line. The strip line/microstrip line MS is formed on the surface of an inner layer of a ceramic package that houses filter chips. FIGS. 9B and 9C show lumped-parameter phase shifters. Those phase shifters are formed with integrated passive devices (IPD), or with chip components (such as chip capacitors and chip inductors) FIG. 9D shows a resonator S1 to which a parallel inductor L1 is connected. The resonator S1 may be a surface acoustic wave resonator, a boundary acoustic wave resonator, or a piezoelectric thin-film resonator, for example. The parallel inductor L1 may be an IPD or a chip component.
The balun for unbalanced-to-balanced conversion illustrated in the circuits of FIGS. 1 and 2 normally has high insertion loss and is large in size, which adds to the costs and the weight. To counter this problem, Japanese Unexamined Patent Publication Nos. 2000-114917 and 2002-359542 disclose the technique of containing a balun for unbalanced-to-balanced conversion in an unbalanced filter or an unbalanced duplexer. With this arrangement, a smaller-sized, light-weight balanced filter and duplexer can be provided.
FIG. 10 is a circuit diagram of a balanced filter. Series-arm resonators S1 through S3 are connected in series, and parallel-arm resonators P1 and P2 are connected in parallel between an unbalanced input terminal In and an unbalanced output terminal T1. With this arrangement, the unbalanced ladder filter 10 is formed. A balun 40 includes the unbalanced terminal T1 and two balanced terminals T21 and T22. A signal that is input through the unbalanced terminal T1 is output from the two balanced terminals T21 and T22. A first capacitor CS is connected in series between the unbalanced terminal T1 and the balanced terminal T21, and a first inductor LP is connected to the balanced terminal T21 and the ground. A second inductor LS is connected in series between the unbalanced terminal T1 and the balanced terminal T22, and a second capacitor CP is connected to the balanced terminal T22 and the ground. With this structure, a small-sized, low-cost filter and duplexer can be provided, and such a filter and duplexer can be connected directly to a balanced LNA and a balanced mixer.
However, where balanced filters are formed with the structures disclosed in Japanese Unexamined Patent Publication Nos. 2000-114917 and 2002-359542, the following two problems are caused. One of the two problems is that the insertion loss of the balun is high, and the filters have high insertion loss accordingly, because lumped parameter devices with low Q values are employed. In Japanese Unexamined Patent Publication Nos. 2000-114917 and 2002-359542, the inductors of baluns are formed with bonding wires, surface acoustic wave resonators, or piezoelectric resonators, and the capacitors are formed with surface acoustic wave resonators or piezoelectric thin-film resonators. In cases where bonding wires, surface acoustic wave resonators, or piezoelectric thin-film resonators are used as inductors or capacitors, the Q values are normally very low. Accordingly, even if a balun is structured with bonding wires, surface acoustic wave resonators, or piezoelectric thin-film resonators, the insertion loss becomes high, and it is unlikely to be able to achieve the required performance as a portable telephone system.
The second problem is that the balance characteristics as a balanced filter or duplexer are poor. The “balance characteristics” include the amplitude difference and phase difference between the differential signals that are output from the two balanced output terminals of a balance filter, which are referred to as the amplitude balance and the phase balance, respectively. The amplitude balance is best at 0 dB, and the phase balance is best at 180 degrees. In a case of a balanced filter having a balun connected to an unbalanced filter (the balanced filter will be hereinafter referred to as the balun-containing balanced filter), the balance characteristics of the balanced filter are basically the balance characteristics of the contained balun. In general, a lumped parameter balun does not excel in balance characteristics, having the amplitude balance of ±0.5 dB and the phase balance of 180 degrees ±5 degrees.
Furthermore, where the inductors of a lumped parameter balun are formed with bonding wires, surface acoustic wave resonators, or piezoelectric thin-film resonators, and the capacitors are formed with surface acoustic wave resonators or piezoelectric thin-film resonators as in Japanese Unexamined Patent Publication Nos. 2000-114917 and 2002-359542, the balance characteristics deteriorate further, due to the frequency dependence of the inductance value and the capacitance value. The amplitude balance becomes ±1 dB, and the phase balance becomes 180 degrees ±10 degrees. With such balance characteristics, high performance of a high-frequency IC to be connected in a later stage cannot be guaranteed.